1. Field of the Invention
The present invention relates to a recording medium, a color conversion table creating apparatus and a color conversion table creating method.
2. Description of Related Art
Conventionally, in order to adjust the output color of a printer to a desired color, an ICC (International Color Consortium) profile, etc. is used. When an ICC profile is created, a LUT (Look Up Table) of “color value to device value” needs to be created in order to convert a color value (L*a*b*, etc.) which does not depend on the device to a device value (CMYK value) which the printer is to output. This LUT is called B to A table in the ICC profile. The B to A table outputs a chart including a plurality of color patches with a printer and is created based on the device value corresponding to each color patch and the color value obtained by measuring the color of each color patch.
However, when a general profile chart such as ISO 12642 is used, since a plurality of combinations of the CMYK value in which the K value is different with respect to a measured color value is possible, even if the device value (CMYK value) is calculated back from the color value using the colorimetric result of the chart, the device value cannot be uniquely settled.
Therefore, there is a method used in which a unique chart fixing the CMYK value so that the color value does not overlap is output and the LUT of the “color value to device value” is calculated back using the colorimetric value (see Japanese Patent Application Laid-Open Publication No. 2004-56271). With this method, since a unique chart is used, the colorimetric value cannot be used in or compared with other systems. Also, the device value which can be used in the chart is fixed, and it is not possible to add a value where the ratio of the K value and the CMY value is changed within the sufficient inner part (not in the vicinity of the border of inside and outside of the color gamut but a portion where the color can be sufficiently reproduced) of the color gamut which can have a plurality of device values.
Also, there is a proposed method in which the color value obtained from measuring the color of the color patch is first converted to the appropriate device CMY value and then the CMY value is converted to the CMYK value. The K value is not used when the color value is converted to the device CMY value and therefore the K value is uniquely determined. However, experience and trial and error is necessary to convert the CMY value to CMYK value while accurately maintaining the color value.
Therefore, a method is proposed which uses a general purpose chart such as an ISO 12642 and uses virtual CMY coordinates determined by a virtual CMY axis (virtual basic color axis) as an intermediate field when calculating (see Japanese Patent No. 2898030). In the virtual CMY coordinate, the color value is corresponded to the selected device value (CMYK value) so as not to overlap and further the color value (L*a*b*, etc.) of the color patch output at each device value is also corresponded. Then, the virtual CMY coordinate corresponding to each color value which is to be described in the LUT of the “color value to device value” is calculated back and the device value corresponding to each color value is obtained through the virtual CMY coordinate.
According to the method described in Japanese Patent No. 2898030, the continuity is emphasized, and therefore, as for the CMY, the device CMY value is corresponded to the virtual CMY coordinate as is. In other words, as the coordinate of the virtual CMY axis increases, only the device value corresponding to the axis increases. Also, as for K, the device K value is determined according to the minimum value of the virtual CMY coordinate. Specifically, the larger the minimum value of the virtual CMY coordinate is, the larger the device K value is.
However, among the device values which compose the color patch on the chart, the device value (and the colorimetric value corresponding to the device value) which is not corresponded to the virtual CMY coordinate cannot be used in the output of the printer. In other words, the range of the color gamut which can be used in the printer changes according to how the device value is corresponded to the virtual CMY coordinate. Also, within the sufficient inner part of the color gamut which can have a plurality of device values, how the ratio of the K value and the CMY value is selected influences the variation stability and the graininess of the printer and relates to the continuity of the device value when the color is converted, and therefore the gradation (smoothness of change of color) of the output image is influenced. Therefore, how the device value is corresponded to the virtual CMY coordinate is an important factor which influences the final color conversion performance.
Conventionally, the method described in Japanese Patent No. 2898030 has been mainly used to create a profile of a device such as high end proofer which needs highly accurate color matching. Therefore, in order to prevent a false outline or strangeness, continuity of the device CMYK value is necessary and the virtual CMY coordinate is created with emphasis on the continuity of the corresponding device value. The device value corresponded to the virtual CMY coordinate does not use the entire theoretical color gamut the printer has, and results in not being able to use a portion of the color gamut in the vicinity of the most outer periphery of the color gamut (low brightness and high saturation). However, when used in the high end proofer with no limit of amount of color material, there has been no large problem in practical use.
As a color coordinate system to indicate the color value, there are several types such as L*a*b*, L*u*v*, etc., and the method of configuration is different in each system. However, basically, a color value is placed in a space including an axis which represents brightness (L*, etc.) and a plane which represents saturation and hue (a*b* plane, etc.). Generally, the color gamut of a printer, etc. has a wide area on the plane of the saturation and the hue when the brightness is about moderate, and when a certain hue is a target, the lower the saturation is, the larger the brightness area is. In other words, the most outer periphery of the color gamut is a “set of a point where the saturation is highest in each brightness or the point where the brightness is highest or lowest in each hue and saturation”. Also, the vicinity of the most outer periphery of the color gamut includes an area which is slightly on the inner side of the most outer periphery of the color gamut.
In an electrophotographic printer, in most cases, there is a constraint in the apparatus of the amount of toner which can be used (total CMYK value) and in many cases, conversion processing (limit of toner amount) is performed to lower the CMYK value in the data. When limit of toner amount is performed, especially in a case where the total CMYK value is limited to less than 300% or in a case where the device CMY value is reduced more than the device K value when the toner amount is limited, the color gamut which cannot be used becomes even larger (the color gamut which can be used becomes even smaller) and there has been a problem of resulting in reduction of color reproducibility.